Yucca plants

We have some ranch land and the yucca population is extreme, so we would like to kill off some of them. We understand there is a liquid that can be administered directly in the center of the yucca and it will kill it without bothering grasses around it. Can you tell me what it is?

Here is an article that may help, although I differ in opinion with the author on the details of the herbicide treatment. From my experience, you do not need to drill holes in order to get effective treatment with a glyphosate systemic herbicide , just spray or paint it on the cut stumps. And you need to use Roundup Concentrate at full strength, or Roundup super concentrate 50/50 with water. And what isn't mentioned is that you need to apply the material immediately after the final cut is made. After even 20 or 30 minutes it will not be as effective.https://www.gardeningknowhow.com/ornamental/foliage/yucca/removing-yucca.htm

But you are in Aggie country; and the Agrilife extension service has some good experience and info for your region. Here is another chemical control option:

You don't need to cut down the plant in order to use this herbicide.
"Remedy" is a triclopyr-ester herbicide, similar to Garlon 4 Ultra and some other generic formulations. It has powerful surfactants to aid penetration of plant tissues. But those surfactants can volatilize (become a gas) at 80 to 85 degrees atmospheric temperature and become hazardous for inhalation and nearby desirable vegetation - crops and landscape plantings. So read the product label thoroughly and follow precautions and use appropriate personal protection, or hire a professional applicator. If you do it yourself on a hot day, consider using a respirator.